Procedure for measuring thermal energy transported by fluid flow

ABSTRACT

A method is provided for measuring thermal power transferred to or from a flowing fluid, independent of the density and specific heat of the fluid. The method may also measure the thermal power transferred to or from the fluid independent of the flow rate of the fluid itself too. This is accomplished by providing a bypass in the fluid stream, determining thermal power transferred to or from the fluid flowing in this bypass line, and then relating the thermal power transferred in the bypass line to the thermal power of the principal fluid stream itself, by calculating temperatures at various points along the principal fluid stream and along the bypass line.

BACKGROUND OF THE INVENTION

The present invention concerns a procedure for measuring thermal energytransported with the aid of fluid flow. The procedure of the inventionis appropriate for measurement both of the increase and decrease of theheat contents in the heat-transporting fluid. The commonest area ofapplication of the procedure is a commercial area heating distributionnetwork, specifically for each house or for each area. The fluidtransporting thermal energy may be any uniformly flowing fluid asregards its state of aggregation and its chemical composition, its mostimportant characteristic being a high specific heat value.

Present-day heat quantity or thermal energy measurement techniques arebased on separate measurement of a temperature differential ΔT=T₁ -T₂and separate measurement of a volumetric flow V, the rate of change ofthermal energy content then being:

    Q=ρ·CV(T.sub.1 -T.sub.2)

where ρ=density of the fluid and C=specific heat. The measurement ofdensity of the fluid and specific heat is completely omitted nowadays,implying that no provision is made for their variations. If steps aretaken to employ as thermal energy-transporting fluid, water which hasbeen improved with additive substances, it is well conceivable that ρCmay not always be as constant as is the case with pure water.

Of the various quantities to measured, V is substantially less accuratethan ΔT; if endeavours are made to improve the measuring accuracy of V,this leads to very expensive meters, such as e.g. the inductive(magnetic) flow meter, which has a metering error amounting to (±0.5% ofthe reading)+(±0.5% of full scale deflection).

The limitations imposed by area heating technology on the developing ofthe method of measurement--to mention a few of them--are:

the meter must not cause any significant increase in demand of pumpingwork; the upper limit for the pressure drop is quite generally 0.1 bar;

the power consumption of the meter should be minimized, and it must notexceed 0.1% of the thermal energy rate that is being measured;

the price at which the meter sells should be concordant with the savingsregarding errors in the charging, owing to heightened accuracy ofmeasurement.

SUMMARY OF THE INVENTION

The object of the invention is to achieve an improvement of previouslyknown methods for measuring thermal energy transported by fluid flow.The more detailed object of the invention is to teach a procedure whichis independent of the fluid's density and specific heat. Still onefurther object of the invention is to provide a procedure wherein theflow rate need not be measured at all. The rest of the objects of theinvention, and the advantages gained with its aid, with become apparentin the disclosure of the invention.

The objects of the invention are attained by a procedure which is mainlycharacterized in that there is provided in parallel across theconsumption unit (where the thermal energy is withdrawn from the fluidpassing through the incoming line), a fluid by-pass flow through aby-pass line; that into said fluid by-pass flow is introduced ortherefrom withdrawn thermal energy at a given rate; and that in saidincoming line, in said return line and in said by-pass line respectivelyare measured the temperatures of the fluid flowing through said incomingline, the fluid flowing through said return line, and the fluid by-passflow flowing through said by-pass line respectively, whereby the thermalenergy transported with the aid of the fluid flow is measurable by theexclusive aid of the rate at which energy is introduced into the saidfluid by-pass flow and of the said temperature measurements.

A number of significant advantages are gained by using the procedure ofthe invention.

The procedure of the invention contains in actual fact no measurement offlow rate at all, in the conventional sense, instead of which onepermits a minor fluid short-circuit flow past the change-of-energyobject under measurement (a heat exchanger for instance). In theprocedure of the invention, one measures the rate at which the fluid'sheat content changes in the object, merely by measurement ofdifferential temperatures and by those relating to the auxiliary heatingor cooling of the minor shunt flow. The procedure of the invention isindependent of the percent magnitude of the shunt flow, of the densityand specific heat of the fluid, and it is thus understood that these mayvary without in any way deranging the measurement. The shunt flow isessential in the invention presented here, and the independence ofdensity and specific heat which was mentioned is only achieved byheating or cooling the same fluid which serves as the actual vehicleproper. The procedure here described becomes increasingly favourable inthe technical and economic respects with increasing pipeline size andenergy quantity to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The procedure of the invention shall be described in detail withreference being made to the principle solution presented in the figuresof the attached drawings, but to which the invention is not meant to beexclusively confined.

In the drawings:

FIG. 1 is a schematic illustration of the method according to thepresent invention; and

FIG. 2 is also a schematic illustration of the method according to thepresent invention in which a specific element is used in one of thesteps of the method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the embodiment depicted in FIG. 1 of the drawings, the thermal energyflow Q₁ is withdrawn from the flowing fluid V₁, and this thermal flowrate is understood to be the quantity that is being measured. The basicinsight of the present invention teaches that one provides in parallelwith the unit 13 where energy is being consumed at a certain rate (e.g.a heat exchanger), a by-pass flow V₂ for the fluid through a by-passline 14. The fluid return line has been denoted with the referencenumeral 12. In the incoming line 11, in the by-pass line 14 and in thereturn line 12 the fluid temperatures T₁,T₂,T₃,T₄ and T₅ are measured atthe points a,b,c,d and e indicated in the figure. The thermal energyrate that one desires to determine is found with the aid of the saidtemperature measurements and of the rate Q₂ at which thermal energy iscarried into or withdrawn from the by-passing fluid V₂. The procedure ofthe invention is based on the following equations:

    Q.sub.1 =ρ·C·V.sub.1 ·(T.sub.1 -T.sub.2) (1)

    Q.sub.2 =ρ·C·V.sub.2 ·(T.sub.4 -T.sub.5) (2)

    ρ·C·(V.sub.1 +V.sub.2)·T.sub.3 =ρ·C·V.sub.2 ·T.sub.5 +ρ·C·V.sub.1 ·T.sub.2      (3)

If these equations are solved for Q₁ as a function of Q₂ and of thetemperatures T₁,T₂,T₃,T₄ and T₅, one finds: ##EQU1## As can be seen fromequation (4), the fluid density ρ and specific heat C of the fluidcancel out, whereby the method is unresponsive to the potentialfluctuations of these quantities. Owing to the symmetric nature ofequation (4), the temperature dependence of ρC is nearly completelyeliminated. Potential changes of the ratio of distribution, k=V₂ /V₁,will cause no measuring error either, because this ratio is measuredwith the aid of differential temperatures (Equation 3). However the flowdistribution ratio has practical significance in that it determinessubstantially the increase of the fluid pumping power required toachieve transport of thermal energy at the rate Q₁. (It should be notedthat the bypass flow is an extra flow made necessary by the procedure.)As readable from equation (4), the inaccuracy of measurement of thepresent procedure arises from the error in measurement of fourdifferential temperatures and from the error in measuring the rate ofthermal energy Q₂. When the by-pass flow is made small, the differentialtemperature T₃ -T₂ will be small. Since Q₂ represents that energy whichthe measuring process consumes, it should also be minimized, wherebythen the differential temperature T₄ -T₅ will also be small. Allconsidered, the differential temperatures T₃ -T₂ and T₄ -T₅ are small,whereby they are the main sources of error in the measuring procedure.The error incurred in the measuring of Q₂ is essentially dependent onthe method by which Q₂ is introduced and transported to be incorporatedin the by-pass flow. Since in actual fact T₁ and T₄ are identical, fourtemperature measuring pick-ups are required in the procedure.

When the flow distribution ratio k(=V₂ /V₁) is low and Q₂ is soregulated that T₄ -T₅ ≈T₃ -T₂, then is it possible to say that the power(energy rate) needed for measurement is Q₂ ≈k². Q₁, and T₃ -T₂ ≈T₄ -T₅≈k. (T₁ -T₂), and the total volumetric flow rate (i.e., the pumpingrequirement) has increased by the factor 1+k.

In frequent instances, e.g. in area heating energy transmission, thedifferential temperature T₁ -T₂ is about 50° C. If it is possible tomeasure the differential temperatures T₄ -T₅ and T₃ -T₂ accuratelyenough even when they are about 1° C., the by-pass flow ratio might evenbe as low as 1/50. Hereby, thus, the pumping requirement would onlyincrease by 2% and the temperature of the return water would be about 1°C. higher than in the case that Q₁ were measured by conventionalprocedures. Q₂ would only amount to 0.04% of Q₁, whereby for instance ifQ₁ were 1 MW, then Q₂ would only be 400 W. This energy, too, will bereturned to the power plant and partly utilized. The temperaturedependence of ρC introduces, with the parameters of this example, acorrection coefficient amounting to a few tenths of one percent at themost and which is dependent on the temperature values used, with agentle slope only.

The differential temperatures may be measured by any method in commonuse. However, attention should be paid to making the temperature pick-upmounting such that the said temperature represents, as well as possible,the average fluid temperature over the whole pipe cross section at thepoint of measurement in question. This is particularly important in themeasurement of the temperature T₃, since if the mixing of the flows V₁and V₂ is not quite thorough, even remarkable temperature gradients maybe encountered.

The measuring of the power rate Q₂ depends on the way in which energy isintroduced in the by-pass flow or if in fact Q₂ is negative, withdrawnfrom it.

For instance if Q₂ is introduced into the by-pass flow by means of aheater resistance therein installed, it will suffice for the measurementof Q₂ if the electric power uptake p of the heater resistance ismeasured, which can be done with adequate accuracy by well-establishedprocedures. Then, thus: ##EQU2## In principle, at least, energy may becarried into the by-pass flow or taken therefrom, by a heat conductor aswell (see FIG. 2). It is possible in that case to measure the power rateQ₂ in the form of the differential temperature ΔT_(R).sbsb.th buildingup across a given thermal resistance R_(th). In that case, the entiremeasuring of thermal energy rate Q₁ would reduce to measurement oftemperatures exclusively, and then: ##EQU3## If the flow distributionratio can be assumed to be known, measuring the temperature T₃ becomessuperfluous. Then: ##EQU4## The effects which the zero point creep ofthe temperature pick-ups has on the critical differential temperaturesT₃ -T₂ and T₄ -T₅ may be eliminated, as required. When Q₂ =0, T₄ -T₅ hasto be zero. Similarly, for V₂ =0, T₃ -T₂ has to be zero. This implies aremarkable simplification of the requirements to be imposed on thedifferential temperature meters in question.

In the foregoing merely the principle solution of the invention has beenpresented, and it is obvious to a person skilled in the art that detailsof the invention may vary in numerous different ways within the scope ofthe inventive idea expressed in the attached claims.

We claim:
 1. A method for measuring thermal power Q₁ transferred to orfrom a fluid flowing through a consumption unit at a volumetric flowrate V₁, comprising the steps of:connecting a bypass line in parallelwith a line carrying said fluid through said consumption unit andallowing a portion of said fluid to flow through said bypass line at avolumetric flow rate V₂, transferring additional thermal power Q₂ to orfrom said fluid flowing through said bypass, measuring temperature, T₁,of said fluid flowing in said consumption unit line as said fluid flowsinto said consumption unit before the transfer of Q₁, measuringtemperature, T₂, of said fluid flowing in said consumption unit line assaid fluid flows out of said consumption unit after the transfer of Q₁,measuring temperature, T₄, of fluid flowing through said bypass as saidfluid flows into said bypass before the transfer of Q₂, measuringtemperature, T₅, of said fluid flowing through said bypass as said fluidflows out of said bypass after the transfer of Q₂, measuring transfer ofQ₂ to or from said fluid flowing through said bypass, measuringtemperature, T₃, of fluid flowing in a return line downstream of saidconsumption unit and bypass lines, and calculating the transfer of Q₁ toor from said fluid flowing through said consumption unit, based on thefollowing formula: ##EQU5##
 2. The method of claim 1 in which theadditional thermal power Q₂ is introduced by a heater resistancedisposed in the bypass line.
 3. The method of claim 2 in which Q₂ iscalculated by measuring electrical power, p, supplied to said heaterresistance.
 4. The method of claim 1 in which the additional thermalpower Q₂ is introduced or withdrawn by a heat conductor disposed in thebypass line.
 5. The method of claim 4 in which Q₂ is calculated bymeasuring a temperature differential across a given length of saidconductor between parallel surfaces normal to the heat flux.
 6. Themethod of claim 1 which comprises the additional steps ofmaintaining aratio, k, of volumetric flow rates V₂ /V₁ relatively low, regulating thetransfer of Q₂ so that T₄ -T₅ is roughly equal to T₃ -T₂, anddetermining Q₁ as roughly equal to Q₂ /k².
 7. A method for measuringthermal power Q₁ transferred to or from a fluid flowing through aconsumption unit at a volumetric flow rate V₁, comprising the stepsof:connecting a bypass line in parallel with a line carrying said fluidthrough said consumption unit and allowing a portion of said fluid toflow through said bypass line at a volumetric flow rate V₂, transferringadditional thermal power Q₂ to or from said fluid flowing through saidbypass, measuring temperature, T₁, of said fluid flowing in saidconsumption unit line as said fluid flows into said consumption unitbefore the transfer of Q₁, measuring temperature, T₂, of said fluidflowing in said consumption unit line as said fluid flows out of saidconsumption unit after the transfer of Q₁, measuring temperature, T₄, offluid flowing through said bypass as said fluid flows into said bypassbefore the transfer of Q₂, measuring temperature, T₅, of said fluidflowing through said bypass as said fluid flows out of said bypass afterthe transfer of Q₂, measuring transfer of Q₂ to or from said fluidflowing through said bypass, calculating the ratio of, k, of volumetricflow rates V₂ /V₁ of fluid flowing through said bypass line to fluidflowing through said consumption unit line, and calculating the transferof Q₁ to or from said fluid flowing through said consumption unit, basedon the following formula; ##EQU6##
 8. The method of claim 7 in which theadditional thermal power Q₂ is introduced by a heater resistancedisposed in the bypass line.
 9. The method of claim 8 in which Q₂ iscalculated by measuring electrical power, p, supplied to said heaterresistance.
 10. The method of claim 7 in which the additional thermalpower Q₂ is introduced or withdrawn by a heat conductor disposed in thebypass line.
 11. The method of claim 10 in which Q₂ is calculated bymeasuring a temperature differential across a given length of saidconductor between parallel surfaces normal to the heat flux.